Ed Harbur and Jim Becker have come up with some great designs for fuel shutoff valve tools. Here's how they did it in their own words and photos.
Ed Harbur's design... I made myself a tool for the L shaped shut off valve. It's very simple - a 3/4" x 10" piece of aluminum rod with with a 5/16" wide x 1/2" deep x 1/2" long slot milled up one side (not across the end) with a through hole at the end of the slot. Works well enough, and quick and easy to make, even for a very amateur machinist like myself.
Jim Becker's design... It is made from 1 foot of 1/2 PVC pipe. The first end shown is for use with "L" type handles. Drill a 1/4 hole on one side of the pipe 1/4 from the end of the pipe. Then use a hacksaw to cut it into a slot. The second end is for use with "T" type handles. Drill a 7/16 hole clear through that just touches the end of the pipe. Then use a hacksaw to cut both holes into slots. Since it is so close to the end, it may be easier to drill a hole a little bit from the end then cut the excess off or just cut slots with a hack saw.